1. Field of the Invention
This invention concerns polymeric binders for fibers and the use of such binders in binding particles to fibers. In particular embodiments, it concerns binding superabsorbent particles to cellulosic fibers which may then be used, for example, to make absorbent fibers that are incorporated into cellulosic products.
2. General Discussion of the Background
Superabsorbent polymers have been developed in recent years that are capable of absorbing many times their own weight of liquid. These polymers, which are also known as water insoluble hydrogels, have been used to increase the absorbency of sanitary products such as diapers and sanitary napkins. Superabsorbent polymers are often provided in the form of particulate powders, granules, or fibers that are distributed throughout absorbent cellulosic products to increase the absorbency of the product. Superabsorbent particles are described, for example, in U.S. Pat. Nos. 4,160,059; 4,676,784; 4,673,402; 5,002,814; and 5,057,166. Products such as diapers that incorporate absorbent hydrogels are shown in U.S. Pat. Nos. 3,669,103 and 3,670,731.
One problem with the use of superabsorbents is that the superabsorbent material can be physically dislodged from the cellulosic fibers of an absorbent product. Separation of the superabsorbent from its substrate reduces the absorbency of the product and diminishes the effectiveness of the superabsorbent material. This problem was addressed in European Patent Application 442 185 A1, which discloses use of a polyaluminum chloride binder to bind an absorbent polymer to a fibrous substrate. The polyaluminum binder, however, suffers from the drawback of being an inorganic product that is not readily biodegradable. Moreover, that European patent does not offer any guidance for selecting binders other than polyaluminum chloride that would be useful in binding absorbent particles.
A method of immobilizing superabsorbents is disclosed in U.S. Pat. No. 4,410,571 in which a water swellable absorbent polymer is converted to a non-particulate immobilized confluent layer. Polymer particles are converted to a coated film by plasticizing them in a polyhydroxy organic compound such as glycerol, ethylene glycol, or propylene glycol. The superabsorbent assumes a non-particulate immobilized form that can be foamed onto a substrate. The individual particulate identity of the superabsorbent polymer is lost in this process. The confluent nature of the superabsorbent material can also result in gel blocking, in which absorption is diminished as the water swollen polymers block liquid passage through the film layer.
U.S. Pat. Nos. 4,412,036 and 4,467,012 disclose absorbent laminates in which a hydrolyzed starch polyacrylonitrile graft copolymer and glycerol mixture is laminated between two tissue layers. The tissue layers are laminated to each other by applying external heat and pressure. The reaction conditions form covalent bonds between the tissue layers that firmly adhere the tissue layers to one another.
Numerous other patents have described methods of applying binders to fibrous webs. Examples include U.S. Pat. Nos. 2,757,150; 4,584,357; and 4,600,462. Such binders are not described as being useful in binding particulates, such as superabsorbent particles, to fibers. Yet other patents disclose crosslinking agents such as polycarboxylic acids that form covalent intrafiber bonds with individualized cellulose fibers, as in European Patent Application 440472 A1; European Patent Application 427 317 A2; European Patent Application 427 316 A2; and European Patent Application 429 112 A2. The covalent intrafiber bonds are formed at elevated temperatures and increase the bulk of cellulose fibers treated with the crosslinker. The covalent bonds between the fibers produce a pulp sheet that is more difficult to compress to conventional pulp sheet densities than in an untreated sheet. Any covalent crosslink bonds that form between the fibers and particles occupy functional groups that would otherwise be available for absorption, hence absorption efficiency is decreased.
It is an object of this invention to provide an improved method of binding particulates, such as superabsorbent particles, to fibers.
It is another object to provide an improved method of binding particulates such that they can be distributed throughout a fibrous product without necessarily being confined to the surface of a product.
Another object of the invention is to provide improved fiber and absorbent products in which particulates are firmly bound to cellulose fibers such that the particles are less likely dislodged by mechanical forces.
Yet another object of the invention is to provide an improved particle binder that is environmentally compatible and easily biodegradable.
Even yet another object is to provide such a product that has improved processing characteristics, such as ease of densification.
Finally, it is an object of the invention to bind a broad variety of particles to many different kinds of fibers using an improved, simple and versatile binding process.